STES (Seasonal Thermal Energy Storage) is, in most cases, too expensive to really be practical because of the huge quantity of the storage medium to supply the seasonal demand. Typically, you loose am good portion of the heat through conduction or have top invest heavily in insulated containment. I find it difficult enough designing thermal stores for demand balancing over a 24 hour period!
BTES (Borehole Thermal Energy Storage) is the most realistic approach. GSHP (Ground Source Heat Pump) is arguably the most efficient method of providing domestic heating. Boreholes are commonly used as a source of heat for GSHP. A common problem with GSHP is that material surrounding the borehole can cool down over several years causing the heat pump to become less efficient and even unusable due to freezing. Using the borehole for passive cooling in the summer will put heat back into the borehole. Putting excess solar thermal heat into the borehole during the summer can also help. Even where ground conditions aren't particularly good for retaining the heat, you can still use the borehole for passive cooling and be assured that it borehole will function well as a heat source. Combining GSHP with solar thermal also the problem of stagnation when the thermal store inside the house can't take any more heat. It's quite a good idea to size the solar thermal array to put roughly the same amount of heat into the ground in the summer as is taken out in the winter.
I wouldn't dig the borehole yourself, that's definitely a job for the professional!